Aqueous ink, ink cartridge and inkjet recording method

Abstract

To provide an aqueous ink for an inkjet capable of recording an image excellent in a color developability by suppressing a bronze phenomenon derived from carbon black.SOLUTION: There is provided an aqueous ink for an inkjet which contains a self-dispersing carbon black and resin particles formed of an acrylic resin. The carbon black has a DBP oil absorption of 120 mL / 100 g or more. The acrylic resin has a glass transition temperature of 30°C or more, the content (mass%) of the resin particles is 0.10 times or more and 2.0 times or less in a mass ratio to the carbon black content (mass %).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an aqueous ink, an ink cartridge, and an inkjet recording method. [Background technology]

[0002] In recent years, inkjet recording methods have seen increased use in printing business documents containing text and diagrams onto recording media such as plain paper. For such applications, water-based inks capable of recording images with excellent color reproduction and durability are required. Pigment inks, which use pigments as colorants, are often used when recording images with superior color reproduction and other characteristics. In black inks, carbon black is primarily used as the pigment. Improving the color reproduction of images recorded with carbon black pigment inks is a particularly important challenge, and various studies have been conducted to date.

[0003] For example, an aqueous inkjet ink containing self-dispersing carbon black with high DBP oil absorption and a large structure has been proposed (Patent Document 1). In addition, an inkjet black ink containing carbon black, resin particles having a blue dye, and resin particles having a fluorescent whitening agent has been proposed (Patent Document 2). It is claimed that using this black ink, the bluish fluorescence of the fluorescent whitening agent and the yellowish-red bronze light derived from the carbon black are additively mixed to approach white, suppressing the bronzing phenomenon and improving the color reproduction of the image. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2017-136846 [Patent Document 2] Japanese Patent Publication No. 2019-143095 [Overview of the project] [Problems that the invention aims to solve]

[0005] The present inventors investigated the aqueous ink proposed in Patent Document 1. As a result, they found that while it was possible to record images with a certain degree of good color development, the bronzing phenomenon caused by the carbon black present on the surface of the recording medium meant that the improvement in color development was not necessarily sufficient. Furthermore, the present inventors also investigated the black ink proposed in Patent Document 2. As a result, they found that when a self-dispersing pigment of carbon black with a large structure was used, the bronzing phenomenon could not be sufficiently suppressed, and the improvement in color development was insufficient.

[0006] Therefore, an object of the present invention is to provide an aqueous inkjet ink capable of recording images with excellent color development by suppressing the bronzing phenomenon caused by carbon black. Another object of the present invention is to provide an ink cartridge using this aqueous ink and an inkjet recording method. [Means for solving the problem]

[0007] In other words, the present invention provides an inkjet water-based ink containing self-dispersing carbon black and resin particles formed from acrylic resin, characterized in that the DBP oil absorption amount of the carbon black is 120 mL / 100 g or more, the glass transition temperature of the acrylic resin particles is 30°C or higher, and the content (mass%) of the resin particles is 0.10 to 2.0 times the mass ratio of the carbon black content (mass%). [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an aqueous inkjet ink that can suppress the bronzing phenomenon caused by carbon black and record images with excellent color development. Furthermore, according to the present invention, it is possible to provide an ink cartridge using this aqueous ink and an inkjet recording method. [Brief explanation of the drawing]

[0009] [Figure 1] It is a cross-sectional view schematically showing one embodiment of the ink cartridge of the present invention. [Figure 2] It is a diagram schematically showing an example of an inkjet recording apparatus used in the inkjet recording method of the present invention. (a) is a perspective view of a main part of the inkjet recording apparatus, and (b) is a perspective view of a head cartridge.

Mode for Carrying Out the Invention

[0010] Hereinafter, the present invention will be described in more detail by giving preferred embodiments. In the present invention, when the compound is a salt, although the salt dissociates into ions and exists in the ink, for convenience, it is expressed as "containing a salt". In addition, the aqueous ink for inkjet may be simply referred to as "ink". Physical property values are values at normal temperature (25°C) unless otherwise specified.

[0011] The bronze phenomenon that occurs in an image recorded with an ink containing carbon black has conventionally been a problem that has been mainly studied in a shiny image recorded on a recording medium having surface gloss such as glossy paper. As a result of the study by the present inventors, it has been found that the bronze phenomenon also occurs in an image that does not show gloss recorded on a recording medium having no surface gloss such as plain paper, and this is one of the factors causing a decrease in color development property.

[0012] The bronze phenomenon refers to a phenomenon in which the reflected light appears to be a color different from the original color due to the refractive index on the particle surface of the pigment existing on the recording medium having wavelength dependence. Therefore, it is considered that the bronze phenomenon occurs more prominently as the exposure amount of the particle surface of the pigment is larger on the surface of the recording medium.

[0013] The inventors investigated an ink that could suppress the bronzing phenomenon caused by carbon black and record images with high optical density and excellent color development. As a result, they found that a self-dispersing pigment of carbon black with a DBP oil absorption capacity of 120 mL / 100 g or more is used in combination with resin particles formed of acrylic resin with a glass transition temperature of 30°C or higher. Furthermore, they found that it is effective to set the content (mass%) of the resin particles to a mass ratio of 0.10 to 2.0 times the content (mass%) of the carbon black, which led to the present invention.

[0014] Carbon black is dispersed in an aqueous medium in a cluster-like structure formed by several to tens of primary particles linked together. The size of the carbon black structure can be determined by the amount of DBP oil absorbed by the carbon black. Specifically, a higher DBP oil absorption value indicates a larger and bulkier structure. Using a self-dispersing carbon black pigment with a large structure can improve the color reproduction of images. This is thought to be because, after the ink is applied to the recording medium, the evaporation of liquid components such as water in the ink promotes the aggregation of carbon black, and the aggregated carbon black tends to remain on the surface of the recording medium. However, investigations have shown that when carbon black tends to remain on the surface of the recording medium, a bronzing phenomenon originating from carbon black occurs significantly, causing the image to take on a reddish to yellowish tint and hindering the improvement of color reproduction. Therefore, the inventors investigated reducing the amount of pigment exposed on the surface of the pigment layer formed on the surface of the recording medium.

[0015] As a result of their investigation, the inventors have found that the occurrence of the bronzing phenomenon originating from carbon black can be suppressed by adding a specific amount of acrylic resin particles having a glass transition temperature above a certain value to the ink. The inventors hypothesize the following mechanism: The presence of particulate resin within the pigment layer formed on the surface of the recording medium increases the thickness of the pigment layer and decreases the density of the pigment within the pigment layer. This reduces the amount of pigment exposed on the surface of the pigment layer, and it is hypothesized that this suppresses the bronzing phenomenon. In order to obtain the above effect, it is considered necessary for particulate acrylic resin particles to be present within the pigment layer. For this to happen, the glass transition temperature (Tg) of the resin particles must be 30°C or higher. Since the temperature of a typical recording environment is estimated to be at most 30°C, in this invention, the glass transition temperature of the resin particles is specified to be 30°C or higher in order to ensure that the resin particles maintain their particle shape in the recording environment. If the glass transition temperature of the resin particles formed from acrylic resin is less than 30°C, the resin particles will not be able to remain in the pigment layer while maintaining their particulate shape, and the bronzing phenomenon will not be suppressed. Furthermore, the resin particles must be made of acrylic resin. If resin particles made of resins other than acrylic resin, such as wax resin particles, are used, the bronzing effect cannot be suppressed, and the color reproduction of the image will also decrease.

[0016] The resin particle content (mass%) must be between 0.10 and 2.0 times the carbon black content (mass%). If the above mass ratio is less than 0.10, the bronzing effect caused by carbon black cannot be suppressed, and the color reproduction of the image will deteriorate. On the other hand, if the above mass ratio is greater than 2.0, the bronzing effect can be suppressed, but the color reproduction of the image will deteriorate due to the influence of the color of the resin particles.

[0017] The DBP oil absorption capacity of carbon black is 120 mL / 100 g or more. If the DBP oil absorption capacity of carbon black is less than 120 mL / 100 g, aggregation will be weak, and the color reproduction of the recorded image cannot be improved.

[0018] <Water-based ink> The ink of the present invention is an aqueous inkjet ink containing resin particles formed from self-dispersing carbon black and acrylic resin. The DBP oil absorption capacity of the carbon black is 120 mL / 100 g or more, and the glass transition temperature (Tg) of the resin particles is 30°C or higher. The content (mass%) of the resin particles is 0.10 to 2.0 times the mass ratio of the carbon black content (mass%). The components constituting the ink will be described below.

[0019] (Pigment) The ink contains self-dispersing carbon black. Any carbon black suitable for inkjet inks can be used. Examples of carbon black include furnace black, lamp black, acetylene black, channel black, and thermal black. These carbon blacks can be used individually or in combination of two or more. The ink may further contain dyes or other substances for purposes such as color matching.

[0020] Self-dispersing carbon black is a self-dispersing pigment in which hydrophilic groups, such as anionic groups, are directly or via other atomic groups bonded to the surface of carbon black particles. Inks containing self-dispersing pigments are prone to changes in state, such as increased viscosity, pigment aggregation, and aggregation, as liquid components such as water evaporate after being applied to a recording medium. Therefore, using self-dispersing carbon black makes it possible to create an ink that can record images with high optical density and excellent color development. On the other hand, if resin-dispersed carbon black is used instead of self-dispersing carbon black, it is not possible to improve the color development of the recorded image.

[0021] The DBP oil absorption of carbon black is 120 mL / 100g or more, preferably 130 mL / 100g or more. While there is no particular upper limit to the DBP oil absorption, it is preferably 200 mL / 100g or less, and more preferably 180 mL / 100g or less. The DBP (dibutyl phthalate) oil absorption of carbon black can be measured in accordance with ASTM D-2414. The DBP oil absorption of carbon black correlates with the structure of the carbon black. The DBP oil absorption of the carbon black used in the examples described later was measured in accordance with ASTM D-2414.

[0022] Examples of anionic groups bonded directly to the particle surface of carbon black, or via other atomic groups, include carboxylic acid groups, sulfonic acid groups, phosphoric acid groups, and phosphonic acid groups. These anionic groups may form salts. When anionic groups form salts, at least one proton of each of these groups is substituted with a cation. Examples of cations include alkali metal ions, ammonium ions, and organic ammonium ions. Examples of alkali metal ions include lithium, sodium, and potassium ions. Examples of organic ammonium ions include aliphatic amines such as mono- or trialkylamines; and cations or salts thereof of aliphatic alcohol amines such as mono- or trialcanolamines. The anionic groups are preferably in the form of alkali metal salts such as sodium or potassium, or ammonium salts, and are more preferably in the form of alkali metal salts such as sodium or potassium.

[0023] Anionic groups may be directly bonded to the surface of carbon black particles or bonded via other atomic groups (-R-). Examples of other atomic groups (-R-) include alkylene groups such as methylene, ethylene, and propylene; arylene groups such as phenylene, naphthylene, anthracenylene, phenantrenylene, and biphenylene; heteroarylene groups such as pyridylene, imidazoylene, pyrazolylene, pyridinylene, thienylene, and thiazoylene; carbonyl groups; ester groups such as carboxylic acid esters, sulfonic acid esters, phosphate esters, and phosphonic acid esters; imino groups; amide groups; sulfonyl groups; and ether groups. Combinations of these groups are also acceptable. To obtain high color development, it is preferable that the anionic groups bonded directly to the surface of carbon black particles or via other atomic groups are carboxylic acid groups. To obtain higher color development, it is more preferable to use a self-dispersing pigment in which carboxylic acid groups are bonded to the surface of carbon black particles via other atomic groups.

[0024] The carbon black content (mass%) in the ink is preferably 1.0% to 10.0% by mass, based on the total mass of the ink. Furthermore, the volume-based cumulative 50% particle size (D50) of the carbon black is preferably 50 nm to 150 nm. In this specification, when simply referred to as "average particle size," it means "volume-based cumulative 50% particle size (D50)." This "volume-based cumulative 50% particle size (D50)" can be measured using a particle size distribution analyzer based on dynamic light scattering. The BET specific surface area of ​​the carbon black is 200 m². 2 / g or more 400m 2 It is preferable that it be less than or equal to / g, and 220m 2 / g or more 300m 2 It is even more preferable that the amount is less than or equal to / g. The BET specific surface area of ​​carbon black can be measured in accordance with JIS Z 8830:2013 (ISO 9277:2010).

[0025] (Resin particles) The ink contains resin particles formed from acrylic resin. "Resin particles" refers to particles of a particle size that can exist dispersed in an aqueous medium, formed from resin. Resin particles can be manufactured according to known methods. Examples of methods for manufacturing resin particles include emulsion polymerization, pre-emulsion polymerization, seed polymerization, and phase inversion emulsification. It is preferable that the resin particles exist independently of the self-dispersing pigment, without being integrated with it. That is, it is not necessary for the resin particles to encapsulate the self-dispersing pigment or for the resin particles to adhere to and disperse the self-dispersing pigment. The self-dispersing pigment is itself dispersible without the assistance of resin, but this does not exclude the possibility that some of the resin particles may adhere to the self-dispersing pigment.

[0026] Whether a resin is "resin particles" or not can be determined by the following method. First, prepare a liquid containing resin (resin solids content: 10% by mass) that has been neutralized with an alkali (such as sodium hydroxide or potassium hydroxide) with an acid value equivalent to the resin's value. Next, prepare a sample solution by diluting the prepared liquid 10 times (by volume) with pure water. Then, measure the particle size of the resin in the sample solution using dynamic light scattering. If particles with a particle size are measured, the resin can be determined to be "resin particles." A particle size analyzer (for example, product name "UPA-EX150," manufactured by Nikkiso) can be used as the particle size distribution analyzer for dynamic light scattering. The measurement conditions in this case can be, for example, SetZero: 30 seconds, number of measurements: 3, measurement time: 180 seconds. Of course, the particle size distribution analyzer and measurement conditions used are not limited to those described above. The reason for measuring the particle size using neutralized resin is to confirm that particles are still formed even when the resin is sufficiently neutralized and in a state where particle formation is less likely.

[0027] The resin particles must be made of acrylic resin. The glass transition temperature (Tg) of the resin particles is 30°C or higher. Preferably, the glass transition temperature of the resin particles is 150°C or lower, and more preferably 100°C or lower. The glass transition temperature of the resin particles can be measured using a differential scanning calorimetry device or the like. In this invention, the glass transition temperature of the resin particles is the value measured for the resin particles themselves that have been extracted from the ink. The temperature cycle during measurement is preferably set to increase the temperature from 25°C to 200°C at a rate of 10°C / min, decrease the temperature from 200°C to -50°C at a rate of 5°C / min, and increase the temperature from -50°C to 200°C at a rate of 10°C / min.

[0028] The resin particle content (mass%) in the ink is preferably 1.0% to 10.0% by mass, and more preferably 1.5% to 8.0% by mass, based on the total mass of the ink. Furthermore, the resin particle content (mass%) in the ink is expressed as a mass ratio of 0.10 to 2.0 times the carbon black content (mass%). The above mass ratio is preferably 0.20 to 1.0 times, and more preferably 0.30 to 0.80 times. If the above mass ratio is less than 0.20 times, the effect of suppressing the bronzing phenomenon may be slightly reduced, and the color reproduction of the image may also be slightly reduced. On the other hand, if the above mass ratio is greater than 1.0 times, the bronzing phenomenon can be suppressed, but the color reproduction of the image may be slightly reduced due to the influence of the color of the resin particles. The total content (by mass) of pigments and resin particles in the ink is preferably 2.0% by mass or more and 20.0% by mass or less, based on the total mass of the ink, and more preferably 2.0% by mass or more and 10.0% by mass or less. In particular, it is especially preferable that it be 2.0% by mass or more and 7.5% by mass or less.

[0029] The volume-based cumulative 50% particle diameter (D50) of resin particles in the ink is preferably 50 nm to 300 nm, more preferably 50 nm to 250 nm, and particularly preferably 50 nm to 230 nm. If the cumulative 50% particle diameter of resin particles in the ink is less than 50 nm, the formed pigment layer becomes thinner, which may slightly reduce the effect of suppressing the bronzing phenomenon and slightly reduce the color reproduction of the image. On the other hand, if the cumulative 50% particle diameter of resin particles in the ink is greater than 300 nm, the bronzing phenomenon can be suppressed, but the effect of improving the color reproduction of the image may slightly decrease due to the effect of light scattering.

[0030] The cumulative 50% particle size by volume of the resin particles is preferably 0.50 to 2.0 times the cumulative 50% particle size by volume of the carbon black. If the above ratio value is outside the specified range, the formed pigment layer will be thinner, which may slightly reduce the bronzing effect and also slightly reduce the color development of the image.

[0031] The acid value of the acrylic resin forming the resin particles is preferably 5 mg KOH / g or more and 100 mg KOH / g or less, and more preferably 5 mg KOH / g or more and 30 mg KOH / g or less. Furthermore, the weight-average molecular weight of the acrylic resin forming the resin particles is preferably 1,000 or more and 2,000,000 or less. The density of anionic groups in the resin particles (number of moles of anionic groups per unit surface area) is 1 μmol / m². 2 More than 500μmol / m 2 Preferably, it is 1 μmol / m³ 2 More than 50μmol / m 2 The following is even more preferable:

[0032] As the acrylic resin that forms the resin particles, it is preferable to have hydrophilic units and hydrophobic units as constituent units. In this specification, "unit" of a resin refers to a unit structure derived from one monomer. Among these, a resin having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one of a (meth)acrylic acid ester monomer and a monomer having an aromatic ring is preferred.

[0033] A hydrophilic unit is a unit having a hydrophilic group, such as an anionic group. Hydrophilic units can be formed, for example, by polymerizing hydrophilic monomers having a hydrophilic group. Specific examples of hydrophilic monomers having a hydrophilic group include acidic monomers having a carboxylic acid group, such as (meth)acrylic acid, itaconic acid, maleic acid, and fumaric acid; and anionic monomers such as anhydrides and salts of these acidic monomers. Cationic ions that form salts include alkali metals such as lithium, sodium, and potassium; ammonium; and organic ammonium ions. Among these, alkali metal ions such as potassium are preferred.

[0034] A hydrophobic unit is a unit that does not have hydrophilic groups such as anionic groups. Hydrophobic units can be formed, for example, by polymerizing hydrophobic monomers that do not have hydrophilic groups such as anionic groups. Specific examples of hydrophobic monomers include (meth)acrylic acid ester monomers such as ethyl (meth)acrylate, methyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; and monomers having aromatic rings such as styrene, α-methylstyrene, and benzyl (meth)acrylate.

[0035] It is preferable that the resin particles have a cross-linked structure. Resin particles with a cross-linked structure tend to remain in the pigment layer while maintaining their particulate shape more easily, thus further suppressing the bronzing phenomenon. To incorporate a cross-linked structure into the resin particles, monomers having two or more polymerizable functional groups, such as ethylenically unsaturated bonds, in their molecule can be used. Specific examples of monomers having two or more polymerizable functional groups in their molecule include diene compounds such as butadiene and isoprene; polyfunctional (meth)acrylates such as 1,4-butanediol di(meth)acrylate, (poly)ethylene glycol di(meth)acrylate, and trimethylolpropane tri(meth)acrylate; and divinylbenzene. If resin particles without a cross-linked structure are used, the effect of suppressing the bronzing phenomenon may be slightly reduced, and the color development of the image may also be slightly reduced.

[0036] The acrylic resin forming the resin particles preferably has units derived from a reactive surfactant. Resin particles formed from an acrylic resin having units derived from a reactive surfactant are further stabilized in their dispersion state due to repulsion caused by steric hindrance. Therefore, using resin particles formed from an acrylic resin having units derived from a reactive surfactant can improve the ink ejection stability. In contrast, using resin particles formed from an acrylic resin that does not have units derived from a reactive surfactant may slightly reduce the ink ejection stability.

[0037] As reactive surfactants, it is preferable to use compounds in which polymerizable functional groups such as (meth)acryloyl groups, maleyl groups, vinyl groups, and allyl groups are bonded to the interior or terminals of molecules composed of hydrophilic and hydrophobic parts. Examples of hydrophilic parts include polyalkylene oxide chains such as ethylene oxide chains and propylene oxide chains. Examples of hydrophobic parts include alkyl groups, aryl groups, and combinations thereof.

[0038] (A salt formed by the bonding of a monovalent cation and an anion) The ink preferably further contains a salt. "Salt" means a compound formed by the combination of a monovalent cation and an anion. By including a salt in the ink, aggregation of the pigment on the recording medium can be promoted, thus further improving the color development property of the image.

[0039] Examples of the monovalent cation include alkali metal ions, ammonium ions, and organic ammonium ions. Examples of the alkali metal ions include lithium ions, sodium ions, and potassium ions. Examples of the organic ammonium ions include alkylamines having 1 to 3 carbon atoms such as methylamine and ethylamine; alkanolamines having 1 to 4 carbon atoms such as monoethanolamine, diethanolamine, and triethanolamine; and the like cations. Among them, alkali metal ions are preferred, and potassium ions are particularly preferred.

[0040] Examples of the anion include Cl - , Br - , I - , ClO - , ClO2 - , ClO3 - , ClO4 - , NO2 - , NO3 - , SO4 2- , CO3 2- , HCO3 - , HCOO - , (COO - )2, COOH(COO - ), CH3COO - , C2H4(COO - )2, C6H5COO - , C6H4(COO - )2, PO4 3- , HPO4 2- , and H2PO4 - and the like can be mentioned.

[0041] Examples of salts composed of a cation and anion bonded together include MCl, MBr, MI, MClO, MClO2, MClO3, MClO4, MNO2, MNO3, M2SO4, M2CO3, MHCO3, HCOOM, (COOM)2, COOH(COOM), CH3COOM, C2H4(COOM)2, C6H5COOM, C6H4(COOM)2, M3PO4, M2HPO4, and MH2PO4, where M represents a monovalent cation. Among these, potassium chloride, sodium acetate, sodium benzoate, potassium benzoate, ammonium benzoate, trisodium citrate, potassium phthalate, and ammonium phthalate are preferred, with potassium phthalate being particularly preferred. By including potassium phthalate, the ink ejection stability can be improved compared to other salts.

[0042] The salt content (by mass) in the ink is preferably 0.05% by mass or more and 1.0% by mass or less, and more preferably 0.1% by mass or more and 0.5% by mass or less, based on the total mass of the ink.

[0043] (aqueous medium) The ink is an aqueous ink containing at least water as an aqueous medium. The ink may further contain a water-soluble organic solvent as an aqueous medium. Deionized water or ion-exchanged water is preferred as the water. The water content (mass%) in the ink is preferably 10.0% by mass or more and 90.0% by mass or less, and more preferably 50.0% by mass or more and 90.0% by mass or less, based on the total mass of the ink. Any water-soluble organic solvent commonly used in inks can be used. Examples include alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing compounds, and sulfur-containing compounds. The water-soluble organic solvent content (mass%) in the ink is preferably 3.0% by mass or more and 50.0% by mass or less, based on the total mass of the ink.

[0044] (Other additives) In addition to the components described above, the ink may also contain, as necessary, water-soluble organic compounds that are solid at room temperature (25°C), such as polyhydric alcohols like trimethylolpropane and trimethylolethane, and urea derivatives like urea and ethyleneurea. Furthermore, the ink may also contain, as necessary, various additives such as water-soluble resins like acrylic resin and urethane resin, surfactants, pH adjusters, rust inhibitors, preservatives, fungicides, antioxidants, reduction inhibitors, evaporation accelerators, and chelating agents. The content (mass%) of these additives in the ink is preferably 0.05% by mass or more and 10.0% by mass or less, and more preferably 0.2% by mass or more and 5.0% by mass or less, based on the total mass of the ink. However, when using water-soluble acrylic resin, it is preferable not to include too much of it. The content (mass%) of water-soluble acrylic resin in the ink is preferably 0.1% by mass or more and 1.5% by mass or less, and more preferably 0.1% by mass or more and 1.1% by mass or less, based on the total mass of the ink. Furthermore, water-soluble urethane resin is effective in improving the scratch resistance of images. The content (mass%) of water-soluble urethane resin in the ink is preferably 0.1% to 5.0% by mass, and more preferably 0.1% to 2.0% by mass, based on the total mass of the ink.

[0045] (Ink properties) The dynamic surface tension of the ink at a lifespan of 10 milliseconds is preferably 40 mN / m or more, and more preferably 45 mN / m or more. When the dynamic surface tension of the ink at a lifespan of 10 milliseconds is 40 mN / m or more, the penetration of the ink in the thickness direction of the recording medium is moderately suppressed, and images with better color development can be recorded. The dynamic surface tension of the ink at a lifespan of 10 milliseconds is preferably 50 mN / m or less. The dynamic surface tension of the ink can be easily controlled by appropriately setting the type and content of the surfactant and water-soluble organic solvent contained in the ink. In order to make the dynamic surface tension of the ink at a lifespan of 10 milliseconds 40 mN / m or more, it is preferable not to use water-soluble organic solvents with low surface tension, or if they are used, to not increase their content.

[0046] The dynamic surface tension of ink at a lifespan of 10 milliseconds can be measured by the maximum bubble pressure method. The maximum bubble pressure method measures the maximum pressure required to release bubbles generated at the tip of a probe (a thin tube) immersed in the liquid being measured, and determines the surface tension of the liquid from this maximum pressure. In the maximum bubble pressure method, the lifespan is the time from the point at which a new bubble surface is generated at the tip of the probe until the maximum bubble pressure (the point at which the radius of curvature of the bubble equals the radius of the probe tip) is reached. The dynamic surface tension of ink in this specification is the value measured at 25°C.

[0047] The viscosity of the ink at 25°C is preferably 1.0 mPa·s to 10.0 mPa·s, and more preferably 1.0 mPa·s to 5.0 mPa·s. The static surface tension of the ink at 25°C is preferably 30 mN / m to 45 mN / m. The pH of the ink at 25°C is preferably 5.0 to 10.0.

[0048] <Ink Cartridge> The ink cartridge of the present invention comprises ink and an ink storage section for storing this ink. The ink stored in this ink storage section is the aqueous ink of the present invention as described above. Figure 1 is a schematic cross-sectional view showing one embodiment of the ink cartridge of the present invention. As shown in Figure 1, an ink supply port 12 for supplying ink to the recording head is provided on the bottom surface of the ink cartridge. The inside of the ink cartridge is an ink storage section for storing ink. The ink storage section consists of an ink storage chamber 14 and an absorbent storage chamber 16, which are in communication with each other via a communication port 18. The absorbent storage chamber 16 is also in communication with the ink supply port 12. Liquid ink 20 is stored in the ink storage chamber 14, and absorbent materials 22 and 24 that hold the ink in an impregnated state are stored in the absorbent storage chamber 16. The ink storage section may not have an ink storage chamber for storing liquid ink, and the entire amount of ink to be stored may be held by an absorbent. Alternatively, the ink storage section may not have an absorbent, and the entire amount of ink may be stored in a liquid state. Furthermore, the ink cartridge may be configured to include an ink storage section and a recording head.

[0049] <Inkjet recording method> The inkjet recording method of the present invention is a method of recording an image on a recording medium by ejecting the aqueous ink of the present invention described above from an inkjet recording head. Methods for ejecting the ink include methods that impart mechanical energy to the ink and methods that impart thermal energy to the ink. In the present invention, it is particularly preferable to employ a method that ejects the ink by imparting thermal energy to the ink. Aside from using the ink of the present invention, the steps of the inkjet recording method may be those of known origin. Even with a simple apparatus configuration that does not have drying means, etc., by using the ink of the present invention, the bronzing phenomenon caused by carbon black can be suppressed and an image with excellent color development can be recorded. For this reason, in the inkjet recording method of the present invention, there is no need to perform a step of drying the image with heat or air.

[0050] Figure 2 is a schematic diagram showing an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, where (a) is a perspective view of the main part of the inkjet recording apparatus and (b) is a perspective view of the head cartridge. The inkjet recording apparatus of the embodiment shown in Figure 2 is equipped with a serial recording head. However, the inkjet recording apparatus used in the inkjet recording method of the present invention is not limited to the embodiment shown in Figure 2, and may be an inkjet recording apparatus equipped with a line recording head. The inkjet recording apparatus of the embodiment shown in Figure 2 is provided with a transport means (not shown) for transporting the recording medium 32, and a carriage shaft 34. A head cartridge 36 can be mounted on the carriage shaft 34. The head cartridge 36 comprises recording heads 38 and 40 and is configured to hold an ink cartridge 42. While the head cartridge 36 is transported along the carriage shaft 34 in the main scanning direction, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. Then, the recording medium 32 is transported in the sub-scanning direction by the transport means (not shown), and an image is recorded on the recording medium 32. In the present invention, it is preferable to use an inkjet recording device equipped with a serial recording head. Furthermore, any recording medium may be used as the recording medium to be recorded using the ink of the present invention, but it is preferable to use a paper-based recording medium that has permeability, such as plain paper or a recording medium having a coated layer (glossy paper or art paper). Among these, it is particularly preferable to use a recording medium without a coated layer, such as plain paper. [Examples]

[0051] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way by the following examples unless it exceeds the gist of the invention. Unless otherwise specified, amounts of components indicated in "parts" and "%" are based on mass.

[0052] Average particle size of pigment (D pThe cumulative 50% particle diameter (based on volume) was measured using a dynamic light scattering particle size distribution analyzer (product name "UPA-EX150", manufactured by Nikkiso). The measurement conditions were: SetZero: 30 seconds, Number of measurements: 3, Measurement time: 180 seconds.

[0053] <Preparation of Pigment Dispersion> (Pigment dispersions 1-4, 6-8, 10) A solution was prepared by dissolving 5.0 g of concentrated hydrochloric acid in 5.5 g of water and cooled to 5°C. At this state, 1.6 g of the treatment agent shown in Table 1 was added. The container of this solution was placed in an ice bath and stirred to maintain the solution temperature below 10°C. A solution prepared by dissolving 1.8 g of sodium nitrite in 9.0 g of ion-exchanged water at 5°C was added. After stirring for 15 minutes, 6.0 g of carbon black with the properties shown in Table 1 was added under stirring, and the mixture was stirred for another 15 minutes to obtain a slurry. The obtained slurry was filtered through filter paper (product name "Standard Filter Paper No. 2", manufactured by Advantec), the particles were thoroughly washed with water, and the mixture was dried in an oven at 110°C. After replacing the counterions from sodium ions to potassium ions by ion exchange, an appropriate amount of ion-exchanged water was added to adjust the pigment content. In this way, each pigment dispersion with a pigment content of 10.0% was obtained. Average particle size D of the pigment P Table 1 shows the size (nm) and the structure of the functional groups on the surface of the pigment particles.

[0054] (Pigment dispersion 5) Carbon black (specific surface area 260 m²) 2 A pigment (D) with a DBP oil absorption capacity of 140 mL / 100 g was added to deionized water and thoroughly stirred. An appropriate amount of sodium hypochlorite (effective chlorine concentration 4%) was added dropwise, and the mixture was stirred at 100°C for 10 hours to allow the reaction to proceed. After the reaction was complete, the mixture was purified by ultrafiltration, and the pH was adjusted to 7.5 using potassium hydroxide. Furthermore, an appropriate amount of deionized water was added to adjust the pigment content. In this way, a pigment dispersion 5 with a pigment content of 10.0% was obtained. Average particle size of the pigment D P The wavelength was 115 nm. This pigment is a self-dispersing pigment in which -COOK groups are bonded to the surface of carbon black particles.

[0055] (Pigment dispersion 9) Carbon black (specific surface area 260 m²) 2 A mixture was obtained by mixing 15.0 parts of (140 mL / 100 g) of a resin dispersant aqueous solution, 30.0 parts of an aqueous solution of resin dispersant, and 55.0 parts of deionized water. As the aqueous solution of the resin dispersant, a styrene / acrylic acid copolymer, which is a water-soluble resin, was dissolved in deionized water using sodium hydroxide in an equimolar amount with the acid value, and an aqueous solution with a resin content of 20.0% was used. This styrene / acrylic acid copolymer had a composition (mol) ratio of styrene to acrylic acid of 33:67, a weight-average molecular weight of 10,000, and an acid value of 200 mg KOH / g. The obtained mixture was placed in a sand grinder and dispersed for 1 hour. After removing coarse particles by centrifugation, it was pressure filtered through a microfilter (manufactured by Fujifilm) with a pore size of 3.0 μm. Then, an appropriate amount of deionized water was added to adjust the pigment content. In this way, a pigment dispersion 9 with a pigment content of 10.0% and a resin dispersant content of 6.0% was obtained. Average particle size D of the pigment P It was 115 nm.

[0056] (Pigment dispersion 11) 15.0 parts of carbon black (product name "Monac 1100" (manufactured by Cabot)), 30.0 parts of a 25.0% aqueous solution of resin dispersant, and 50.0 parts of deionized water were placed in a glass container filled with 0.3 mm diameter zirconia beads at a 50% density. As the aqueous solution of the resin dispersant, a styrene / acrylic acid copolymer (product name "Joncryl 690" (manufactured by BASF), a water-soluble resin, was neutralized with potassium hydroxide to a molar ratio of 0.85 based on the acid group. This styrene / acrylic acid copolymer had a weight-average molecular weight of 16,500 and an acid value of 240 mgKOH / g. The contents of the glass container were mixed for 15 hours using a simple disperser (product name "DAS200-K" (manufactured by LAU) to disperse the carbon black. After removing aggregated components by centrifugation at 5,000 rpm for 30 minutes, an appropriate amount of deionized water was added to adjust the pigment content. In this way, a pigment dispersion 11 was obtained having a pigment content of 15.0% and a resin dispersant content of 7.5%. Average particle size D of the pigment P It was 80nm.

[0057] TIFF2023048994000001.tif95170

[0058] <Preparation of resin particles> (Measurement conditions for physical properties) The glass transition temperature of the resin particles was measured according to the following procedure. First, the dispersion of resin particles was heated to 60°C and dried, then sealed in an aluminum container to prepare the sample. Next, using a differential scanning calorimetry device (product name "DSC Q1000", manufactured by TA Instruments), the sample was heated to 200°C at a rate of 10°C / min, cooled to -50°C at a rate of 5°C / min, and then heated to 200°C at a rate of 10°C / min to measure the glass transition temperature (°C). The average particle size (D) of the resin particles was also measured. R The cumulative 50% particle diameter (based on volume) was measured using a dynamic light scattering particle size distribution analyzer (product name "UPA-EX150", manufactured by Nikkiso). The measurement conditions were: SetZero: 30 seconds, Number of measurements: 3, Measurement time: 180 seconds.

[0059] (Resin particles 1-16, 18) The emulsions of monomers of the types and amounts shown in Table 2 were added dropwise to a solution obtained by mixing the amounts of deionized water and 0.1 parts of potassium persulfate shown in Table 2 under a nitrogen atmosphere, and the polymerization reaction was carried out at 80°C with stirring. After cooling to 25°C, potassium hydroxide equivalent to the acid value of the resin and an appropriate amount of deionized water were added to obtain a dispersion of resin particles with a resin particle content of 20.0%. The properties of the resin particles in the obtained dispersion are shown in Table 2. The abbreviations for monomers in Table 2 are: MMA: methyl methacrylate, BMA: n-butyl methacrylate, EMA: ethyl methacrylate, MAA: methacrylic acid, AA: acrylic acid, BDDMA: 1,4-butanediol dimethacrylate, KH-05: reactive surfactant (product name "Aqualon KH-05", manufactured by Daiichi Kogyo Seiyaku). The reactive surfactant (product name "Aqualon KH-05", manufactured by Daiichi Kogyo Seiyaku) is a surfactant having a structure in which an allyl group (a polymerizable functional group) is introduced into the basic skeleton of a polyoxyethylene alkyl ether sulfate salt.

[0060] (Resin particles 17) A dispersion of resin particles 17, containing 20.0% resin particles, was obtained using a wax (resin particles) copolymer of 1-octene and maleic anhydride. Resin particles 17 had a glass transition temperature (Tg) of 80°C and an average particle diameter of 170 nm. The properties of the resin particles in the obtained dispersion are shown in Table 2.

[0061] (Resin particles 19) A solution was obtained by dissolving 2.0 parts of CI Solvent Blue 70 (product name "Orasol Blue 855", manufactured by BASF) and 8.0 parts of styrene-acrylic acid copolymer (product name "Joncryl 611", manufactured by BASF) in 40.0 parts of ethyl acetate. The obtained solution was added to a solution in which 0.15 parts of sodium dodecyl sulfate was dispersed in 90.0 parts of deionized water and stirred. An emulsion was obtained by emulsifying with an ultrasonic homogenizer (product name "Advanced Digital Sonifier 250DA", manufactured by BRANSON) at an amplitude of 50% for 10 minutes. Ethyl acetate was removed from the emulsion obtained using an evaporator. After cooling, the solution was pressure filtered through a pore size 1.2 μm filter (product name "HDCII", manufactured by Pall), and an appropriate amount of deionized water was added. In this way, a dispersion of resin particles 19 with a resin particle content of 10.0% was obtained. The encapsulation rate of blue dye in the resin particles 19 was 20.0%. Table 2 shows the properties of the resin particles in the obtained dispersion.

[0062] (Resin particles 20) A dispersion of resin particles 20 was obtained in the same manner as described above for resin particles 19, except that CI fluorescein brightening agent 184 (trade name "Uvitex OB", manufactured by BASF) was used instead of CI solvent blue 70. The resin particle content in the obtained dispersion was 10.0%, and the encapsulation rate of the fluorescent whitening agent (CI fluorescein brightening agent 184) in the resin particles 20 was 20.0%. The properties of the resin particles in the obtained dispersion are shown in Table 2.

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[0064] <Ink preparation> Each ink was prepared by mixing the components (unit: %) shown in the upper section of Tables 3-1 to 3-4, thoroughly stirring and dispersing them, and then pressure filtering through a 2.5 μm pore size polypropylene filter (Pall). The values ​​listed for "polyethylene glycol" in Tables 3-1 to 3-4 are the number-average molecular weights, and "acetylenol E100" is the trade name for a nonionic surfactant manufactured by Kawaken Fine Chemicals. The characteristics of each prepared ink are shown in the lower section of Tables 3-1 to 3-4. The dynamic surface tension of the ink was measured at a lifespan of 10 msec using a dynamic surface tension measuring device (trade name "Bubble Pressure Tensiometer BP2", KRUSS) based on the maximum bubble pressure method.

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[0067] TIFF2023048994000005.tif174170

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[0069] <Rating> The following items were evaluated using the prepared ink. For image recording, an inkjet recording device (product name "GX6030", manufactured by Canon) equipped with a recording head that ejects liquid by the action of thermal energy was used. In this example, the recording duty cycle of a solid image recorded under the condition that two ink droplets with a mass of 11.7 ng ± 10% per drop are applied to a unit area of ​​1 / 600 inch × 1 / 600 inch is defined as 100%. In this invention, "AA", "A", and "B" were defined as acceptable levels and "C" as unacceptable levels in the evaluation criteria for each item shown below. The evaluation results are shown in Table 4.

[0070] (Color development) A 2cm x 2cm solid image with a 100% recording duty cycle was recorded onto three recording media (plain paper, product name "CS-064F A4", manufactured by Canon). After one day, the optical density of the solid image was measured using a fluorescence spectrometer (product name "FD-7", manufactured by Konica Minolta) under the following conditions: illumination: M1 (D50), observation light source: D50, field of view: 2°. The average optical density of the images recorded on the three recording media was then calculated, and the color reproduction of the images was evaluated according to the evaluation criteria shown below. AA: The average optical density was 1.46 or higher. A: The average optical density was between 1.43 and 1.46. B: The average optical density was 1.40 or higher and less than 1.43. C: The average optical density was less than 1.40.

[0071] (Bronze resistance) Ten different 2cm x 2cm solid images were recorded on a recording medium (plain paper, product name "CS-064F A4", manufactured by Canon) with recording duty cycles ranging from 10% to 100% in 10% increments. After one day, the solid images with recording duty cycles that showed a bronzing effect (changing from red to yellow) were visually inspected, and the bronzing resistance of the images was evaluated according to the evaluation criteria shown below. Generally, the higher the recording duty cycle, the greater the amount of reflected light from the image, making the bronzing effect more likely to occur. In other words, the higher the recording duty cycle at which the bronzing effect occurs, the better the image's bronzing resistance. A: The bronzing phenomenon did not occur in any of the recording duty cycles of the solid images. B: The bronze effect occurred in solid images with a recording duty cycle of 70% or more. C: The bronze effect occurred in solid images where the recording duty cycle was less than 70%.

[0072] (Discharge stability) Ten 2cm x 2cm solid images with a 100% recording duty cycle were recorded onto a recording medium (plain paper, product name "CS-064F A4", Canon). Then, the nozzle check pattern of the GX6030 was recorded onto the same type of recording medium. Next, 3,000 solid images were recorded under the same conditions, and the nozzle check pattern was recorded again. The nozzle check pattern after 10 recordings and the nozzle check pattern after 3,000 recordings were compared, and the ink ejection stability was evaluated according to the evaluation criteria shown below. A: Nozzle check patterns were recorded correctly both after 10 images and after 3,000 images. B: The nozzle check pattern was recorded normally after 10 images, but there was distortion in the nozzle check pattern after 3,000 images. C: Irregularities were found in both the nozzle check pattern after recording 10 images and after recording 3,000 images.

[0073] TIFF2023048994000007.tif140170

[0074] This embodiment includes the following configurations and methods. (Composition 1) An aqueous inkjet ink containing resin particles formed from self-dispersing carbon black and acrylic resin, The DBP oil absorption capacity of the carbon black is 120 mL / 100 g or more. The glass transition temperature of the resin particles is 30°C or higher. A water-based ink characterized in that the content (mass%) of the resin particles is 0.10 to 2.0 times the mass ratio of the carbon black content (mass%). (Configuration 2) The aqueous ink according to Configuration 1, wherein the content (mass%) of the resin particles is 0.20 times or more and 1.0 times or less in mass ratio to the content (mass%) of the carbon black. (Configuration 3) The aqueous ink according to Configuration 1 or 2, wherein the carbon black is a self-dispersing pigment in which anionic groups are bonded directly to the surface of carbon black particles or via other atomic groups. (Configuration 4) The aqueous ink according to any one of Configurations 1 to 3, wherein the carbon black is a self-dispersing pigment in which a carboxylic acid group is bonded directly to the surface of the carbon black particles or via other atomic groups. (Configuration 5) The aqueous ink according to any one of Configurations 1 to 4, wherein the carbon black is a self-dispersing pigment in which a carboxylic acid group is bonded to the surface of carbon black particles via other atomic groups. (Configuration 6) The aqueous ink according to any one of Configurations 1 to 5, wherein the resin particles have a crosslinked structure. (Configuration 7) The aqueous ink according to any one of claims 1 to 6, wherein the cumulative 50% particle diameter of the resin particles on a volume basis is 50 nm or more and 300 nm or less. (Configuration 8) The aqueous ink according to any one of Configurations 1 to 7, wherein the cumulative 50% particle diameter of the resin particles on a volume basis is 0.50 times or more and 2.0 times or less in ratio to the cumulative 50% particle diameter of the carbon black on a volume basis. (Configuration 9) The aqueous ink according to any one of Configurations 1 to 8, wherein the acrylic resin has a unit derived from a reactive surfactant. (Configuration 10) An aqueous ink according to any one of Configurations 1 to 9, wherein the dynamic surface tension at a lifespan of 10 milliseconds is 40 mN / m or more. (Composition 11) Furthermore, an aqueous ink according to any one of Compositions 1 to 10, which contains potassium phthalate. (Configuration 12) An ink cartridge comprising ink and an ink storage section for storing the ink, An ink cartridge characterized in that the ink is the water-based ink described in any one of the items 1 to 11. (Method 1) An inkjet recording method in which ink is ejected from an inkjet recording head to record an image on a recording medium, An inkjet recording method characterized in that the ink is an aqueous ink according to any one of the items 1 to 11 of the configuration.

Claims

1. A water-based inkjet ink containing self-dispersing carbon black and resin particles formed from an acrylic resin, The carbon black has a DBP oil absorption of 120 mL / 100 g or more, The glass transition temperature of the resin particles is 30°C or higher, the content (mass%) of the resin particles is 0.10 times or more and 2.0 times or less in mass ratio to the content (mass%) of the carbon black, The water-based ink is characterized in that the carbon black is a self-dispersing pigment in which an anionic group is bonded to the particle surface of the carbon black via another atomic group.

2. 2. The aqueous ink according to claim 1, wherein the content (% by mass) of the resin particles is 0.20 to 1.0 times the content (% by mass) of the carbon black in terms of a mass ratio.

3. 3. The water-based ink according to claim 1, wherein the carbon black is a self-dispersing pigment in which a carboxylic acid group is bonded to the particle surface of the carbon black via another atomic group.

4. The water-based ink according to claim 1 or 2, wherein the resin particles have a crosslinked structure.

5. An aqueous ink as described in claim 4, wherein the resin particles are crosslinked with a polyfunctional (meth)acrylate.

6. 3. The aqueous ink according to claim 1, wherein the resin particles have a volume-based cumulative 50% particle diameter of 50 nm or more and 300 nm or less.

7. 3. The aqueous ink according to claim 1, wherein the volume-based cumulative 50% particle diameter of the resin particles is 0.50 to 2.0 times the volume-based cumulative 50% particle diameter of the carbon black.

8. 3. The aqueous ink according to claim 1, wherein the acrylic resin has a unit derived from a reactive surfactant.

9. 3. The water-based ink according to claim 1, wherein the ink has a dynamic surface tension of 40 mN / m or more at a life time of 10 ms.

10. An aqueous ink according to claim 1 or 2, having a dynamic surface tension of 45 mN / m or more at a lifetime of 10 ms.

11. An aqueous ink according to claim 1 or 2, wherein the dynamic surface tension at a lifetime of 10 ms is 50 mN / m or less.

12. The water-based ink according to claim 1 or 2, further comprising potassium phthalate.

13. An aqueous ink as described in claim 1 or 2, wherein the content (mass%) of the resin particles is 0.30 times or more and 0.80 times or less in mass ratio to the content (mass%) of the carbon black.

14. An aqueous ink described in claim 1 or 2, wherein the DBP oil absorption of the carbon black is 180 mL / 100 g or less.

15. An aqueous ink described in claim 1 or 2, wherein the glass transition temperature of the resin particles is 150°C or less.

16. An aqueous ink described in claim 1 or 2, wherein the glass transition temperature of the resin particles is 100°C or less.

17. An aqueous ink as described in claim 1 or 2, wherein the carbon black content (mass %) is 1.0 mass % or more and 10.0 mass % or less, based on the total mass of the ink.

18. An aqueous ink as described in claim 1 or 2, wherein the content (mass %) of the resin particles is 1.0 mass % or more and 10.0 mass % or less, based on the total mass of the ink.

19. An ink cartridge comprising ink and an ink storage section for storing the ink, 3. An ink cartridge, wherein the ink is the water-based ink according to claim 1.

20. An inkjet recording method for recording an image on a recording medium by ejecting ink from an inkjet recording head, 3. An ink-jet recording method, wherein the ink is the aqueous ink according to claim 1.

Images